The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to a phantom for quantitative diffusion MRI.
Among numerous magnetic resonance imaging contrast mechanisms, the insight that diffusion-weighted MRI provides into the microstructural changes in tissues makes it an excellent biomarker for the detection and evaluation of necrosis, infection, fibrosis, and cancer. To realize the diagnostic potential of diffusion-weighted MRI, it is important to develop quantitative diffusion MRI (“q-DMRI”) techniques that produce reproducible, accurate, and robust measurements of the apparent diffusion coefficient (“ADC”) or diffusivity. Such q-DMRI techniques would have tremendous potential for various imaging applications, including tumor characterization and treatment monitoring. However, for q-DMRI to become a valid biomarker, validation of its accuracy and reproducibility is needed.
Phantoms are devices that are placed in the bore of an MRI system to test or calibrate its operation. Phantoms may be made of materials having known magnetic resonance properties or they may contain cavities filled with such materials. The MRI system is operated with the phantom in place to produce a spectrum or an image from which proper operation of the MRI system may be determined. Phantoms exhibiting desired diffusion behavior could provide a controlled setting for the development and validation of q-DMRI techniques.
Ideally, phantoms used for diffusion quantification should possess a single-peak magnetic resonance spectrum; a single exponential diffusion decay profile; and apparent diffusion coefficient (“ADC”), or diffusivity, values over the entire physiological range (e.g., 0.62×10−3 mm2/s to 2.60×10−3 mm2/s). Furthermore, it would be preferable for the phantoms to have these characteristics under a well controlled temperature.
Early phantoms used in diffusion-MRI were made of different pure substances, such as water or ethanol. These phantoms are easy to construct and easy to use, and they also have well-characterized simple diffusion behavior. Compounds such as ethanol, however, also have a magnetic resonance spectrum with multiple peaks. Multiple spectral peaks can lead to image artifacts, such as chemical shift related artifacts. In addition, by using pure substances, only a limited number of discrete ADC values can be achieved at any controlled temperature.
Recently proposed phantoms for q-DMRI have used water as a solvent, to which solutes are added to change the diffusivity of the resulting solution. As one example, these phantoms have included water solution phantoms in which sucrose or a polymer, such as sucrose polyvinyl pyrrolidone (“PVP”), are added as solutes to water. These water solution phantoms, however, do not meet all of the desirable properties for a q-DMRI phantom listed above.
There remains a need to provide phantoms for q-DMRI that have desirable magnetic resonance spectra, have desirable diffusion decay profiles, and have diffusivity values that span the entire range of diffusivities of biological tissues under a variety of physiological conditions and tissue environments.